Criminals stole sensitive information about roughly 100,000 taxpayers through the Internal Revenue Service’s “Get Transcript” application, a major data breach at the U.S.’s national tax agency.

The thieves first stole information including Social Security details, dates of birth and street addresses from an outside, non-IRS source, the government agency said Tuesday. They then used that information to clear a multistep authentication process and access the IRS site, along with all the personal tax details stored there.

The matter is now under review by the Treasury Inspector General for Tax Administration and the IRS’ Criminal Investigation unit. The Get Transcript application has also been temporarily shut down.

Though roughly 100,000 taxpayers’ information was accessed, the IRS identified 200,000 attempts to tap into such data. It plans to notify all those affected, and will also provide free credit-monitoring services for the 100,000 taxpayers whose accounts were accessed.

The IRS said in a statement is “working aggressively to protect affected taxpayers and continue to strengthen our protocols.”

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Katherine Noyes has been an ardent geek ever since she first conquered Pyramid of Doom on an ancient TRS-80. Today she covers enterprise software in all its forms, with an emphasis on cloud computing, big data, analytics and artificial intelligence.